Advances in network technology have resulted in an enormous number of image files to be managed. There has been available an image search method for searching an image similar to an image serving as a query from the enormous number of images. For selecting an image from the enormous number of images with speed, a search method called BoF (Bag of Features) approach is under development. This approach is based on a document search method called BoW (Bag of Words) model. By BoF approach, feature vectors extracted from a search target image are respectively associated with Visual Words, which correspond to words in the BoW model, and the similar image is searched by using appearance frequency of the Visual Words.
Patent Literature 1 discloses converting image feature amount vectors extracted from an image serving as a query into a smaller number of vectors by using a clustering method, and searching images using the converted vectors as queries.